Modulating system



p 8, 1942- D; e. c. LUCK ,351

I MODULATINGI SYSTEM Filed May 24, 1939 1 Ha. I. I F1 6.2.

Sfiventor Patented Sept. 8, 1942 2,295,351 momma'rmo srs'r'mu David G.0. Luck, Haddon Heights, N. 11, assignor to tion of Delaware RadioCorporation of America, a corpora- Application May 24, 1939, Serial No.275,370

Claims. (01. 250-11) 1 This invention relates to modulating systems, andmore particularly to an improved modulator of the absorption or phaseopposition type in which a constant load impedance is presented to theradio frequency output tubes so that fre-: quency and phase distortionis reduced.

A modulating system has been proposed in.

which the output of an oscillation generator is connected to an antennathrough an impedance inverting network, the antenna having shuntedacross it an impedance which is variable in response to a modulatingvoltage. -The output of the oscillator is divided between the antennaand the shunt impedance in inverse proportion to their instantaneousimpedances. The variable impedance is the absorption modulator fromwhich the system derives its name. Such a system is described andclaimed in a copending application of J. W. Conklin, Ser. No. 187,618,filed January 29, 1938, and assigned to the assignee of the presentapplication. 7

Another system has been proposed in which an oscillation generator isconnected to an antenna, and an auxiliary modulator tube is connectedacross the antenna, as in the case first mentioned, but in this caseradio frequency energy is supplied to the antenna not only by thegenerator, but also through the auxiliary modulator tube. The phase ofthe auxiliary energy with respect to the directly impressed energy isvaried by the modulating voltage so that the resultant radiated energyis a function of the modulating voltage. Such a system is described andclaimed in a copending application of A. W. Vance, Serial No. 210,304,filed May 27, 1938, and assigned to the same assignee as the presentapplication.

In both of the systems briefly described above, the impedance of theload to which the generator is connected varies between wide limits.Thus, during downward modulation, when the absorption modulator istaking the output load, the impedance presented to the generator throughthe impedance inverter is high, while, during upward modulating peaks,when the absorption or modulatortube is biased below plate currentcut-off,

the impedance which is presented to the generator is low. phaseopposition system. 7

As a result of the wide variation of impedance across the generator,phase or frequency modulation is produced and the output tubes of thegenerator are not able to operate at their greatest efllciency over theentire operating cycle. In addition, there is a tendency to feed lowmodulation frequencies back into the power supply.

A similar effect is noted in the It is the primary purpose of thepresent invention to avoid the dlfliculties which have been pointed outabove and to provide an absorption or phase opposition modulator inwhich a constant load impedance is presented to the generator outputamplifier throughout the modulating cycle.

The absorption and phase opposition modulators have another commonfeature. In both instances, modulation is accomplished by dissipatingenergy in a circuit which represents a complete loss insofar as theradiated energy is concerned. Thus, in the absorption modulation system,if the downward modulation of the radiated wave is to be complete, themodulator tubes must absorb and dissipate as heat the output of theoutput amplifier tubes, and for intermediate degrees of modulation atleast some of the generated energy is absorbed and dissipated as heatloss.

In the case of the phase opposition system, complete downward modulationof the radiated wave is obtained when the modulator tube is delivering acurrent of average carrier value, so that it is subject to considerableplate dissipation at a time when no modulated energy is being radiated.

It is, therefore, a further object of this invention to provide amodulation system in which the energy which has been wasted in thedevices of the prior art is utilized, for example, by variably dividinga constant amount of energy between two antenna systems which preferablyhave different radiation patterns. 1

Further objects of this invention and a better understanding of itsnature and operation will be set forth in the following descriptionwhich is to be considered in connection with the accompanying drawing.Its scope is indicated by the appended claims.

Referring to the drawing, Figures 1 and 2 are simplified diagrams to aidin the explanation of the operation of this invention; Figure 3 is ablock diagram illustrating a modulator in accordance with thisinvention; Figures 4 and 5,11- lustrate simplified forms of thisinvention;v Figures 6 and 7 are modifications of the arrangement shownin Figures 4 and 5, and Figure 8 is the circuit diagram of a modulator.

The general arrangement and theory underlying this invention will firstbe explained by referring-to Fig. 1. Four impedances R1, R2, R1 and R2are connected in series-parallel between two terminals 9 and H, theimpedance between which is represented by R.

Writing an expression for the impedance R, it is seen that:

Let R1=R2=Ro, then it follows that and when R1R2=Ro, substituting in(2), then (3) R=Ro (4) Thus, when R1 and R1 are both equal to someconstant R0, and when Ro==R1Rz, from Equation 4, it is seen that theinput impedance R is equal to the constant Ro without regard to theactual value of R1 or R2, so long as they maintain the requiredrelation. It may similarly be shown that the same relation applies tothe circuit arrangement illustrated in Fig. 2 under the same conditions.

If R1 represents the impedance of an antenna, and Re represents theequal impedance of a second similar antenna, or an absorbing network ofidentical characteristics, and if R1 and Re represent variableimpedances which are controlled by'modulating voltages, it is evidentthat if they are varied so that the relation R1F=R1Rz is maintained,then the input impedance R will remain constant.

Referring now to Fig. 3, an arrangement is shown in which impedances R1and R1 have been replaced by antennas I3 and I5, respectively; R1 and R2have been replaced by modulators l1 and I9, respectively; and agenerator 2| has been connected across terminals 9, II. The generator isany suitable radio frequency source such as an oscillator or poweramplifier, or the like. A modulating voltage is applied to the input ofthe two modulators by suitable connections to terminals 23. 25. It isnow necessary to consider means for varying the impedances R1 and R2 ofthe modulators I1 and I9 so that they maintain the relation of Equation3.

Referring now to Fig. 4, an arrangement is shown in which the modulatorimpedance R2 of Fig. 1 has been replaced by a quarter wave line 21 orequivalent impedance inverting network, having at its outer end amodulator 29 whose impedance is equal to R1. The two impedances R1 andR: are equal to the constant impedance R0. Thus, the two modulatorimpedances are alike, and may be varied simultaneously. Thecharacteristic impedance Z of the quarter wave line is made equal to thedesired input impedance of the system R0. Consequently, if the impedancelooking into the quarter wave line, represented by the dotted symbol 3|,is now called Ra, it can be demonstrated that Ro=R1R:', which satisfiesthe relation which must be maintained by the two modulators R1 and .R2'.

In the drawing, impedances R1, R2, or the specific value R representeither the impedance of an antenna, or of a dissipating network. If twoantennas are used they preferably have different directionalcharacteristics so that a modulated field is produced as the output isshifted from one to the other. If a single antenna and a dissipatingnetwork are used, the antenna may be nondirectional, since the totaloutput is divided alternately between a nonradlating network and theantenna.

The fact that modulation is produced is readily understood in connectionwith Figs. 1 or 2,

. the other.

when it is considered that, if the impedance of 7 Various modificationsof circuit arrangements may be employed. two of which are illustrated inFigs. 6 and 7. The arrangements illustrated employ several quarter wavelines, or their equivalent, and may be convenient for various reasons.In Fig. 6 one modulator R1 is connected in series with the antennaimpedance R0 and in series with a quarter wave line 33. A second line 35is connected in series with line 33. The second line is similar to thefirst except that it includes the additional impedance inverter 21.Thus, each antenna, or the antenna and the dissipating network, has beenremoved from the generator terminals 9, ll. Fig.- 7 represents anequivalent arrangement in which similar apparatus is designated asbefore and will usually be the preferred arrangement.

Fig. 8 is the schematic diagram of a modulator which may be employed inconnection with this invention. The modulator comprises a pair of triodetubes 31, 39 whose grid and cathode electrodes are connected together.The input modulating potential is applied between the cathode and gridelectrodes. The anode electrodes are returned to cathode throughsuitable choke coils 43 and 45. The impedance between the two anodeelectrodes is a function of the applied modulating voltage, andcorresponds to the impedance R1. Two such modulators are utilized,illustrated at [1, I 9, in Fig. 3, for example, and their inputs areconnected in parallel. While I have illustrated a specific modulator,this is merely by way of example, as other types will be apparent tothose skilled in the art. Thus, instead of merely varying an impedance,a modulator operating.on the phase opposition principle may be employed,in which radio frequency voltages are produced in series with the twoantennas, the phase of said voltages with respect to that impressedacross the terminals 9, I I being varied by the modulating potential.

While I have explained this invention in its application to amplitudemodulation of a radio frequency carrier, it is not to beso limited. Theconstant impedance system has many applications. For example, it may beused to provide a readily variable source of radio frequency voltage inconnection with a signal generator. The output voltage could be takenacross resistor R1 while resistor R: would be within the device. Becauseof its constant impedance, the tendency to cause the oscillatorfrequency to drift as the output is varied is greatly reduced. Thesystem is useful in many applications which require controlled variationof a current, alternating or direct, in response to a signal. Thecontrolling signal may be either a voice representing voltage or aregular or intermittent impulse. Relays may be operated by the voltagesacross the fixed resistors, and the device is therefore useful as aswitching relay.

I claim:

1. A constant impedance load device including separate current paths,one of said paths comprising serially connected fixed and variableimpedances, and the other of said paths including a fixed impedance anda variable impedance, said fixed impedances having equal values, one ofsaid variable impedances being connected in series with said fixedimpedance through an impedance inverter, and means for varying saidvariable impedances in like sense and amount to wary the relativeimpedances of said paths and to maintain constant the input impedance ofsaid load device.

2. A constant impedance load device comprising two serially connectedload impedances, each of said impedances comprising a fixed and avariable impedance connected in parallel, and means for varying saidvariable impedances in accordance with the equation RiRrs=Ro 1 where R1and R2 are the instantaneous values of the respective variableimpedances, and R0 is the value of each fixed impedance.

3. A device of the character described in claim 2 in which at least oneof said fixed impedances is an antenna.

4. A device of the character describedin claim 2 in which said means forvarying said variable impedances includes a source or modulating voltageand at least one of said fixed impedances is an antenna.

5. A constant impedance load device including parallel current paths,each 01 said paths including in series a fixed and a variable impedance,and means for varying the impedance of said paths in accordance with theequation where R1 is the instantaneous impedance of the variable portionof-one oi said paths, R2 is the instantaneous impedance of the variableportion of the other of said paths, and R0 is the fixed impedanceportion of each or said paths.

DAVID G. C. LUCK.

